The present invention relates to fluids useful as subterranean treatment fluids, and more particularly, to polymeric fluid loss additives, subterranean treatment fluids having improved fluid loss control, and their associated methods of use.
Providing effective fluid loss control for subterranean treatment fluids is highly desirable. “Fluid loss,” as that term is used herein, refers to the undesirable migration or loss of fluids (such as the fluid portion of a drilling mud or cement slurry) into a subterranean formation and/or a proppant pack. The term “proppant pack,” as used herein, refers to a collection of a mass of proppant particulates within a fracture or open space in a subterranean formation. These “treatment fluids” may comprise any fluids used in a subterranean application. As used herein, the term “treatment” does not imply any particular action by the fluid or any component thereof. Treatment fluids may be used in any number of subterranean operations, including drilling operations, fracturing operations, acidizing operations, gravel-packing operations, acidizing operations, well bore clean-out operations, and the like. Fluid loss may be problematic in any number of these operations. In fracturing treatments, for example, fluid loss into the formation may result in a reduction in fluid efficiency, such that the fracturing fluid cannot propagate the fracture as desired.
Fluid loss control materials are additives that lower the volume of a filtrate that passes through a filter medium. Particulate materials may be used as a fluid loss control materials in subterranean treatment fluids to fill the pore spaces in a formation matrix and/or proppant pack and/or to contact the surface of a formation face and/or proppant pack, thereby forming a filter cake that blocks the pore spaces in the formation or proppant pack, and prevents fluid loss therein. However, the use of particulate fluid loss control materials may be problematic. For instance, the sizes of the particulates may not be optimized for the pore spaces in a particular formation matrix and/or proppant pack and, as a result, may increase the risk of invasion of the particulate material into the interior of the formation matrix, which may greatly increase the difficulty of removal by subsequent remedial treatments. Additionally, once fluid loss control is no longer required, for example, after completing a treatment, remedial treatments may be required to remove the previously-placed fluid loss control materials, inter alia, so that a well may be placed into production. However, particulates that have become lodged in pore spaces and/or pore throats in the formation matrix and/or proppant pack may be difficult and/or costly to remove. Moreover, particulate fluid loss control materials may not be effective in low-permeability formations (e.g., formations with a permeability below about 1 milidarcy (“md”)) since the leakoff rate in those formations is not high enough to pull the particulates into the pore spaces or into contact with the surface of the formation face and/or proppant pack so as to block or seal off the pore spaces therein.
Gelled fluids and fluid loss control “pills” comprising high-molecular weight polymers and/or crosslinked polymers have also been used to improve fluid loss control. “Crosslinked polymers” are polymers wherein two or more of the polymer molecules have become “crosslinked” by interaction with a “crosslinking agent,” such as a metal ion or a borate ion. When included in a treatment fluid, these polymeric materials may viscosify that fluid, thereby reducing the leakoff rate of the fluid into the formation and/or proppant pack. Polymer molecules also may reduce fluid loss by filling the pore spaces of the formation matrix and/or proppant pack, thereby preventing the flow of fluid through those pore spaces.
However, the use of polymeric fluid loss control additives also may present a variety of problems. First, in treatments using crosslinked polymers, it may be necessary to maintain certain conditions (for example, specific pH levels) in order for the crosslinking agent to crosslink the polymer molecules, which may require the use of additional additives that add cost and complexity to the operation, or in some cases may be incompatible with other aspects of the treatment fluid or the operation. The polymer molecules also may “over-crosslink” in the presence of high concentrations of crosslinking agent, yielding a treatment fluid that is over-viscosified, difficult to break, exhibits syneresis (i.e., separation of liquid in a gel), or has other undesirable rheological properties.
Also, as in the case of particulate fluid loss control materials, the molecules of polymeric fluid loss control additives may not be sized correctly (e.g., have a desired molecular weight and/or hydrodynamic volume) to effectively fill the pore spaces within a formation matrix and/or proppant pack. The “modality” of a polymeric material is defined herein to refer to the number of ranges that the molecular weights of the molecules of the polymeric material fall within. For example, a “monomodal” polymeric material refers to a polymeric material that comprises molecules that have molecular weight distributions within a single range, whereas a “multimodal” polymeric material refers to a polymeric material that comprises at least two pluralities of polymer molecules having different average molecular weights. The “dispersity” of a polymeric material is defined herein to refer the breadth of the range of molecular weights of the molecules in a given sample of a polymer. The dispersity of a polymeric material may be defined numerically as the breadth of the molecular weight distribution for a sample of a polymer divided by the average molecular weight. Polymeric materials found in nature are generally monomodal and/or have very low dispersities of molecular weight (i.e., they are relatively “monodispersed,” or “narrowly dispersed,” which refers to a polymeric material whose molecules all have a molecular weight that falls within a narrow range). Polymeric materials with relatively low dispersities of molecular weight, like the particulate fluid loss materials discussed above, may not be able to fill the pore spaces sufficiently to prevent fluid loss into the formation. For example, if the polymer molecules are all relatively large, they may be unable to fit within certain pore throats in the formation to plug the pore spaces therein. However, if the polymer molecules used as a fluid loss control additive are all relatively small (e.g., so as to to fill smaller pore spaces in low permeability formations), this may, among other things, limit the particulate transport capability of the fluid (e.g., limiting the size and amount of proppant particulates that the fluid can carry downhole).